1. Technical Field of the Invention
The present invention relates to earth boring bits, and more particularly to those having rotatable cutters, also known as cones.
2. Description of Related Art
Reference is made to U.S. Pat. No. 5,129,471 to Maurstad the disclosure of which is hereby incorporated by reference. Reference is also made to U.S. Pat. No. 3,397,928 to Galle the disclosure of which is hereby incorporated by reference. Reference is still further made to U.S. Pat. No. 4,372,624 to Neilson the disclosure of which is hereby incorporated by reference. Reference is also made to U.S. Pat. No. 3,765,495 to Murdoch the disclosure of which is hereby incorporated by reference.
Earth boring bits with rolling element cutters have bearings employing either rollers as the load carrying element or with a journal as the load carrying element. The use of a sealing means in rock bit bearings has dramatically increased bearing life in the past fifty years.
Early seals for rock bits were designed with a metallic Belleville spring clad with an elastomer, usually nitrile rubber (NBR). The metallic spring provided the energizing force for the sealing surface, and the rubber coating sealed against the metal surface of the head and cone and provided a seal on relatively rough surfaces because the compliant behavior of the rubber coating filled in the microscopic asperities on the sealing surface. Belleville seals of this type were employed mainly in rock bits with roller bearings. The seal would fail due to wear of the elastomer after a relatively short number of hours in operation, resulting in loss of the lubricant contained within the bearing cavity. The bit would continue to function for some period of time utilizing the roller bearings without benefit of the lubricant.
A significant advancement in rock bit seals came when o-ring type seals were introduced. These seals, as disclosed by Galle, were composed of nitrile rubber and were circular in cross section. The seal was fitted into a radial gland formed by cylindrical surfaces between the head and cone bearings, and the annulus formed was smaller than the original dimension as measured as the cross section of the seal. The squeeze of the seal was defined as the percentage reduction of the cross section from its original state to the deflected state. Murdoch disclosed a variation of this seal by elongating the radial dimension which, when compared to the seal disclosed by Galle's, required less percentage squeeze to form an effective seal. Several other minor variations of this concept have been used, each relying on an elastomer seal squeezed radially in a gland formed by cylindrical surfaces between the two bearing elements. Nielson describes what is called a V-ramp seal gland. In this arrangement, the seal is compressed between two concentric surfaces with at least one of the surfaces having a V-shaped cross section. The seal is centrally located in the V-ramp aligned with an axis of symmetry for the surfaces forming the V-shaped cross section.
To minimize sliding friction and the resultant heat generation and abrasive wear, rotating O-rings are typically provided with a minimal amount of radial compression. However, reciprocating seals must have a much larger radial compression to exclude contamination from the sealing zone during axial sliding (typically about twice the compression). The rock bit seal must both exclude contamination during relative head/cone axial motion and minimize abrasive wear during rotation.
A need exists in the art for an improved sealing system for use in rock bits.